1. Field of the Invention
The present invention relates to a circuit and a method of detecting a mirror signal for an optical disc apparatus, and more particularly, to a circuit and a method of accurately detecting a mirror signal during a high-speed seek or a seek, for a high density disc or a poor quality disc.
2. Description of the Related Art
Generally, in an optical disc apparatus which reproduces data recorded on an optical disc, signals generated when a servo jumps tracks to seek a predetermined position include a track cross signal and a mirror signal. The number of tracks is counted using these two signals. Also, the velocity of seeking a track is controlled and the accurate position of the track is sought by using a phase difference between these two signals.
FIG. 1 shows a block diagram of a conventional circuit which detects a mirror signal for an optical disc apparatus. Referring to FIG. 1, a mirror signal detecting circuit 20 selects an RFSUM signal from a signal picked up by a pickup 10, detects a mirror signal by using the RFSUM signal, and supplies the mirror signal to a servo processor 30.
The mirror signal detecting circuit 20 includes an RFSUM signal selector 21, a top envelope detector 22, a bottom envelop detector 23, a top holding unit 24, a bottom holding unit 25, an amplifier and low-pass filter (AMP & LPF) 26, a comparison voltage determiner 27, and a comparator (COMP) 28.
The RFSUM signal selector 21 selects the RFSUM signal. The top envelope detector 22 detects a top envelope of the RFSUM signal. The bottom envelope detector 23 detects a bottom envelope of the RFSUM signal. The top holding unit 24 detects a top level of the top envelope. The bottom holding unit 25 detects a bottom level of the bottom envelope. The AMP & LPF 26 amplifies and low-pass filters the bottom envelope. The comparison voltage determiner 27 determines a comparison voltage. The comparator 28 compares an output of the AMP & LPF 26 with the comparison voltage to output a mirror signal.
The operation of the mirror signal detecting circuit 20 shown in FIG. 1 will be described with reference to FIGS. 2A–2F.
The RFSUM signal selector 21 receives a signal picked up by the pickup 10 and provides the signal in a form of an RFSUM signal shown in FIG. 2A to the top envelope detector 22 and the bottom envelope detector 23. The top envelope detector 22 detects a top envelope of the RFSUM signal and outputs a top envelope signal shown in FIG. 2B. The bottom envelope detector 23 detects a bottom envelope of the RFSUM signal and outputs a bottom envelope signal shown in FIG. 2C. The top holding unit 24 holds the top envelope signal to detect a top level and outputs a top hold signal shown in FIG. 2D. The bottom holding unit 25 holds the bottom envelope signal to detect a bottom level and outputs a bottom hold signal shown in FIG. 2E.
The bottom envelope signal, shown in FIG. 2C, output from the bottom envelope detector 23 is input to the AMP & LPF 26. The bottom hold signal, shown in FIG. 2E, output from the bottom holding unit 25 is input as a reference voltage level to the AMP & LPF 26. The AMP & LPF 26 amplifies and low-pass filters the bottom envelope signal based on the bottom hold signal. At the same time, the comparison voltage determiner 27 controls the top hold signal and the bottom hold signal output from the top holding unit 24 and the bottom holding unit 25 at a predetermined level, respectively, and provides the predetermined level as a comparison voltage of the comparator 28, i.e., a slice level. The comparator 28 compares a voltage output from the AMP & LPF 26 with the comparison voltage determined by the comparison voltage determiner 27 and provides a mirror signal of a digital signal shown in FIG. 2F to the servo processor 30.
In a case where a high density disc or a poor quality disc (a disc that is deflected, eccentric, tilted, has a defect, or the like) is used, the high density disc or the poor disc may be defocused or degraded. In this case, where a seek operation is performed, a deteriorated RFSUM signal is generated as shown in FIGS. 3A through 3D. FIGS. 3A and 3B show the actual waveform of an RFSUM signal deteriorated where a disc is deflected and tilted, FIG. 3C shows the actual waveform of an RFSUM signal deteriorated by fingerprints, and FIG. 3D shows the actual waveform of an RFSUM signal deteriorated due to defects of a disc.
A bottom envelope signal also deteriorate s due to the deteriorated RFSUM signal, i.e., the intensity of the bottom envelope signal is reduced and inconstant. Accordingly, a glitch occurs where the comparator 28 generates a digital signal (logic “high” or logic “low”), and therefore the duty cycle is distorted. As a result, an accurate seek is difficult. To solve these problems, the AMP & LPF 26 has to perform a proper signal amplification and a low-pass filtering. However, since the glitch cannot be completely removed only by a low-pass filtering, the comparator 28 has to properly control the hysteresis and the level of the comparison voltage.
In other words, where a deteriorated RFSUM signal shown in FIG. 4A passes through the top envelope detector 22 and the top holding unit 24, a top envelope signal and a top hold signal are generated as shown in FIG. 4B. Where the deteriorated RFSUM signal passes through the bottom envelope detector 23 and the bottom holding unit 25, a bottom envelope signal and a bottom hold signal are generated as shown in FIG. 4C. The AMP & LPF 26 amplifies the bottom envelope signal according to the voltage level of the bottom hold signal. The comparator 28 slices the amplified bottom envelope signal to a proper reference voltage (slice level) determined by the comparison voltage determiner 27 to detect a mirror signal MIRR.
However, where the bottom envelope signal is low, the bottom envelope signal has to be amplified by a high amplification degree. Where a difference in the bottom envelope signal occurs due to the deviation of a reflectance, depending on types of discs, the amplification degree of the AMP & LPF 26 and the reference voltage level of the comparator 28 have to be set to predetermined values. In the former case, the possibility that the bottom envelope signal is saturated is high. In the latter case, the amplification degree and the reference level have to be fixed to predetermined values.
Only where a signal is amplified based on the center value thereof, the signal is not saturated and can be greatly amplified. Since the mirror signal detecting circuit 20 shown in FIG. 1 amplifies the bottom envelope signal according to a voltage level of the bottom hold signal, a center level of the amplified bottom envelope signal differs greatly from a comparison voltage level (slice level) of the comparator 28. Thus, the amplified bottom envelope signal cannot be properly sliced, as shown in FIG. 4D. Therefore, the comparator 28 cannot detect a proper mirror signal, as shown in FIG. 4E. Also, where the AMP & LPF 26 does not amplify a signal and the comparator 28 does not induce hysteresis to prevent this phenomenon, an inaccurate mirror signal is detected due to noise.